1. Field of the Invention
This invention relates to a fibrous paper cover stock material having a textured surface pattern and a method for manufacturing it.
2. Description of the Related Art
Textured paper cover stocks are conventionally used as covering material in applications such as wall coverings or book covers. In such applications, it is often desirable that the textured paper be identifiable as cloth, leather, or some other textured product. When a textured paper cover stock is successfully substituted for traditional covering materials, significant cost savings results. Paper cover stocks also have the advantage that they can be easily coated with a protective coating that resists moisture and wear. It is frequently difficult to adhere protective coatings to cloth or leather surfaces, and cloth surfaces require a significantly greater quantity of coating per unit area than is the case with paper because cloth has a higher degree of permeability than paper.
A further potential advantage of a paper cover stock over materials like cloth or leather is that the paper cover stock can be manufactured with one textured surface and one smooth surface. The smooth surface can be more easily adhered to flat surfaces like book covers or walls than is the case with traditional covering materials like cloth or leather.
Because of these relative advantages enjoyed by paper cover stock, paper cover stock products having textured surface patterns that resemble cloth or leather have been produced and applied with some success. Such conventional textured cover stock products have been produced by treating finished paper manufactured according to conventional paper making methods. Texturizing of conventionally manufactured paper sheets occurs off-machine, that is, subsequent to the production of the paper itself. According to one conventional texturizing method, paper sheets are subjected to moisture and then run between textured rollers to impart a textured pattern on a surface of the sheets. Alternatively, paper sheets may be run through heated textured rollers to produce a textured pattern on a surface of the sheets. Unfortunately, sheets produced by such off-machine methods are usually readily identifiable as not having the appearance of fabric or leather because it has not been possible to consistently produce a paper product having the undulations and detail necessary to mimic traditional cover material surfaces when off-machine texturizing methods are applied.
In addition, paper products that have been textured off-machine suffer from a "memory effect" when exposed to water vapor or moisture. Upon exposure to water vapor or moisture, conventional off-machine textured sheets tend to return to their original finished state. This memory effect is especially troublesome in bookbinding or wallpapering where aqueous glues are used.
After exposure to water vapor or moisture, paper cover stocks textured off-machine lose much of their surface texture so that any resemblance they may have to traditional cloth or leather covering materials is lost. Thus, the aesthetic value of such products can be radically reduced by moisture.
A further disadvantage of conventional textured cover materials is that off-machine pressing substantially reduces the caliper of a cover stock. Thus, to obtain a desired caliper, it is necessary to use a larger weight of product than would be the case if no off-machine treatment was applied. This can substantially increase the cost of producing a cover stock material of a desired caliper.
Finally, off-machine treatment necessitates additional treatment procedures and equipment. It is preferred that a paper cover stock be in a state ready for sale and shipment at the time it comes off the paper making machine without the need for further time consuming and costly treatment steps.
In the production of bulky paper products, various on-machine treatments have been disclosed. These disclosed on-machine methods are designed to yield bulky products such as paper towels and absorbent tissue. For example, in U.S. Pat. No. 4,102,737 (Morton), a bulky absorbent sheet for use in tissue or toweling is produced when a paper web is predried on a pattered fabric before final drying and creping. As disclosed in Morton, bulky paper sheets are prepared by partially predrying a fibrous web on a drying/imprinting fabric before the web is pressed against the drying/imprinting fabric in a nip formed between a pressure roll and a dryer drum. The nip pressure serves to impress the fabric into the thermally predried paper web. Drying is completed on the dryer drum and creped with a doctor blade upon removal from the drum to obtain greater bulk. Because the web is nearly dry before embossing takes place, a textured pattern capable of mimicking a cloth of leather cover material cannot be achieved by this process.
In U.S. Pat. No. 4,551,199 (Weldon), another process for treating web material is disclosed. According to Weldon, bulking and creping of a paper web is achieved by transporting a web into a differential velocity nip defined by a web support and an open mesh fabric pick-up member having voids therein. The pick-up member has a relative velocity slower than that of the support surface at the nip location. When the web is applied to the fabric pick-up member, the web is impressed into the voids of the fabric to emboss the web. As the web approaches the nip, a deceleration of the web occurs due to the slower moving fabric filaments of the pick-up member causing the web to collapse on itself one or more times to form crepe folds. The succeeding folds in the web press against earlier folds, pushing them into the voids of the fabric. The size and number of folds are determined by the flexibility of the web and the magnitude of the relative velocity differential between the pick-up fabric and the transport member support surface. Sheets produced according to the process disclosed in Weldon have an apparent bulk greater than 0.4 caliper pts/lb ream (that is, an apparent density less than 2.5 lbs ream/caliper pt in mils) which is indicative of folding and bulking much greater than is desirable for cover stock materials.